Pro Comp roller rocker

Dynamic Compression Ratio and how it’s used to select Camshafts

Dynamic Compression Ratio and how it’s used to select Camshafts

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Dynamic compression ratio is the CR that is applied as the piston goes up into the compression stroke. For most OEM and aftermarket camshafts the intake valve is still open and flowing air/fuel mix into the cylinder allowing for more power to be made. For this reason the intake valve remains open longer for larger cam shafts. The flip side of this is the piston has moved up the cylinder several degrees with the valve open so no real compression has taken place until the cam closes the valves. The amount the piston moves after the valve closes calculated against the compression ratio gives you the dynamic compression ration. For big cams an engine can have a static compression ratio of 10:1 but still run fine on 87 octane pump gas cause the dynamic compression ratio is around 8:1 allowing for the use of pump gas.

I will create separate post to show how to calculate the DCR and select camshafts based on these numbers.


Dynamic Compression Ratio Calculators


Each of these calculators and resource links must be taken as a resource for information and not an absolute law or rule. Many of them are accurate for their situation but not always accurate for your unique problems. I am providing this as reference material for others that may want to do further research on the subject. I also did not want to copy and reproduce someones work or images as my own.

This is one of my personal favorites

United Engines & Machine
Wallace Racing Website
Jeep Stroker DCR

Here is some more info and more detailed explanation.

Crankshaft Coalition
Angry Engineer: DCR
Kennedys Dynotune
Classic Inlines SCR & DCR
FTL Racing
Bad Ass Cars: How cams affect compression